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1
Pulse almonds in a food processor until coarsely chopped, then transfer to a small bowl. Pulse walnuts in food processor until finely chopped and set aside.
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2
Finely chop 1 bar of milk chocolate and 1 bar of bittersweet chocolate with a large knife, then combine in a bowl. Repeat with remaining 2 chocolate bars and combine in another bowl.
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3
Butter a large baking sheet and put on a heatproof work surface. Bring butter, sugar, and water to a boil over moderately high heat, whisking until smooth, then stirring occasionally, until mixture is golden and registers 300u00b0F on thermometer, 20 to 30 minutes.
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4
Immediately stir in almonds, then carefully pour mixture onto center of prepared baking sheet (use caution when pouring hot liquids). Spread toffee into a 15- by 12-inch rectangle (1/4 inch thick) with a heatproof spatula, smoothing top, and let stand 1 minute, then sprinkle 1 bowl of chopped chocolate evenly on top of hot toffee. Let stand until chocolate is melted, 4 to 5 minutes. Spread chocolate evenly using a heatproof rubber spatula or an offset metal spatula. Sprinkle half of walnuts evenly over melted chocolate, then cover with a sheet of wax paper or parchment.
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5
Wearing oven mitts, invert second baking sheet over toffee and carefully invert toffee and sheet. If toffee does not release itself, use a spatula to loosen it. Sprinkle remaining chocolate on top to melt, then spread (as above) and sprinkle with remaining walnuts. Place a sheet of wax paper or parchment on top and press lightly to help nuts adhere. Freeze toffee on baking sheet until chocolate is firm, about 30 minutes. Break toffee into pieces, then transfer to an airtight container and keep at room temperature.
